A vinyl chloride resin which has widely been employed as a pliable synthetic resin is known to generate a hazardous material during a combustion process, because of which a development of a substitute of a vinyl chloride resin is desired. One substitute for a pliable vinyl chloride resin is a propylenic polymer. While a propylenic polymer is produced in the presence of various catalyst, a propylenic polymer produced using a conventional catalyst system involves a disadvantageously increased amount of stickiness-causing components as a result of an attempt to impart a pliability (i.e. a low modulus). The increase in the amount of stickiness-causing atactic polypropylenes (hereinafter referred to as APP) leads to a deteriorated surface condition of a molded article obtained.
On the other hand, an application of a molded article in a form of a sheet or a film to a food product or a medical use may involve various problems. Accordingly, a propylenic polymer having a more satisfactorily weighed relationship between a low level of the modulus and the quantity of the stickiness-causing components is desired.
Since a propylenic polymer generally has a greater supercooling degree required for initiating a crystallization when compared with an ethylenic polymer, it provides a resin characterized by a lower crystallization temperature even if it has a same melting point. Accordingly, it may cause a problematically poor molding performance especially with a heat seal grade product having a low melting point. In an attempt to reduced the heat seal temperature, a method for reducing the stereoregularity index of a propylenic polymer is employed, or a copolymer with other olefins is used. Among those produced in such attempt, a conventional low stereoregular propylenic polymer obtained by using a Ziegler catalyst system has a broad stereoregularity distribution, and an attempt to obtain a pliable polymer (i.e. having a low modulus) results in an increase in the amount of stickiness-causing components, including one derived from APP which causes a poor physical property of a low stereoregular propylenic polymer, such as a poor surface condition of a molded article once such propylenic polymer is molded. Thus, it is desired to obtain a film, a fiber, a sheet or a molded article in which a low melting point and a very narrow stereoregularity distribution possessed by a low stereoregular polymer are still preserved and which has an excellent transparency and a low temperature heat seal performance and is highly rigid.
Other disadvantageous characteristics of a propylenic polymer include a high glass transition temperature Tg (about 0° C.), due to which the impact resistance at a low temperature (e.g. −30° C.) is problematically poor.
Recently, an olefin polymer produced by using a metallocene catalyst was also proposed, but a metallocene catalyst has a limited active center, which results in a narrow molecular weight distribution of a polymer obtained, which is suitable to a precise injection molding or an ordinary injection molding and can preferably be employed to mold a fiber, but is not always satisfactory when applied to a heat molding, an extrusion, a blow molding or a molding of a foam or a film. An LLDPE (linear low density polyethylene) obtained using a metallocene also involves the problems of poor transparency and surface condition, although it has a pliability.